A fluid dynamic pressure bearing device is a bearing device in which pressure is generated by a dynamic pressure action of a fluid (lubricating fluid) generated in a bearing gap through relative rotation of a bearing sleeve and a shaft member inserted into an inner periphery of the bearing sleeve, and which supports the shaft member in a non-contact manner with this pressure. The fluid dynamic bearing device is roughly classified into one which includes a dynamic pressure generating portion which positively generates a bearing action to the lubricating fluid at a bearing gap, and other one which does not include such a dynamic pressure generating portion, and in which both the inner peripheral surface of the bearing sleeve and the outer peripheral surface of the shaft member are formed into a cylindrical surface, i.e., “cylindrical bearing.”
The fluid dynamic pressure bearing device is endowed with various features such as high speed rotation, high rotational precision, and low noise, and is widely used, by utilizing its features in recent years, as a bearing device for use in a spindle motor mounted to an information apparatus, for example, a magnetic disk device such as an HDD and an FDD, an optical disk device such as a CD-ROM, a CD-R/RW, or a DVD-ROM/RAM, or a magneto-optical disk device such as an MD or an MO, a polygon scanner motor mounted to a laser beam printer (LBP), etc., a fan motor mounted to a personal computer (PC), etc., or a small motor mounted to electric equipment such as an axial fan.
In this kind of motor for information apparatus, along with increase of information processing amount, laminating of recording medium or attainment of high speed rotation is rapidly progressed. Along with this progress, further enhancement of the rigidity (moment rigidity), in particular, the rigidity with respect to moment load is required to the fluid dynamic bearing device.
As means for enhancing the moment rigidity of the fluid dynamic bearing device, it is generally employed a structure in which the radial bearing portions are formed at two portions in the axial direction while isolating therebetween, to thereby enlarge the bearing span. As this kind of structure, for example, there is known the following structures:
(1) a structure in which the radial bearing surfaces are formed at vertical two positions of a single bearing sleeve (for example, refer to Patent Document 1); and
(2) a structure in which two bearing sleeves are arranged in the axial direction, and one each of the radial bearing surface is formed on both bearing sleeves (for example, refer to Patent Document 2).
Patent Document 1: JP 10-9250 A
Patent Document 2: JP 11-269475 A